7.3 Movements of Earth and
the Moon
• Why is it important to understand the Earth’s
movements?
• Understanding Earth’s rhythms is critical to the
understanding of the life cycles of all living things
• The seasons are a result of Earth’s revolution
around the sun
• Why is it important to understand
the Moon’s movements?
• The Moon is responsible for the tides that occur
on Earth
• The tides are the rising and falling of ocean
waters
• The orbit of the Moon around the Earth results
in the phases of the Moon, as well as eclipses
Earth’s Motion
• As a result of Earth’s rotation
everything in the night sky appears to
rise in an easterly direction and set in
a westerly direction
• As a result of Earth’s revolution
around the Sun, you see different
constellations at different times
of the year
• The path of Earth’s orbit around the Sun is not a
perfect circle, it is an ellipse
• A circle has one centre point
• Whereas, an ellipse has two points called
focal points, which define the shape of the ellipse
• The Sun is at one focal point which defines the
shape of the ellipse
• So, at one focal point in Earth’s elliptical orbit
Earth is closest to the Sun
• And at another point, Earth is farthest from
the Sun
Why do we experience seasons?
• Earth does not rotate in an upright position
• It rotates on an axis tilted at 23.5o from the flat
plane of Earth’s orbit and that tilt does not
change
• In the summer months, the northern hemisphere
is tilted toward the Sun
• In the winter months, the northern hemisphere
is tilted away from the Sun
• As a result of Earth’s tilt, sunlight strikes Earth’s
surface at different angles
• Areas that receive sunlight at larger angles
receive more sunlight for longer periods of time
• Light rays that hit the Earth straight on are called
direct rays
• Light rays that do not hit the Earth straight on
and are slanted to some degree are called
indirect rays
• Refer to “Lesson 14: What causes the change of
seasons?” booklet for more information
The Moon’s Motion
• During the night, the Moon appears to move
across the sky with the stars and the planets
due to Earth’s rotation
• The Moon also appears to move from west to
east as it moves in its orbit
• The Moon makes a
complete orbit around
Earth in about 27.5 days
• As the moon completes one orbit around
Earth, the Moon rotates only once on its axis
• As a result, you always see the same side of
the Moon!
• The other side of the Moon is nicknamed “the
dark side of the Moon” for this reason
• However, the other side of the Moon does
receive sunlight for about two weeks
(therefore, not always “dark”)
Phases of the Moon
• The Moon is always half illuminated by the Sun
• But how much of the illumination you see
depends on where the Moon is relative to Earth
• We classify the amount of illumination we see as
the phases of the Moon
• Phases of the Moon: the monthly progression
of changes in the appearance of the Moon,
which result from different portions of the
Moon’s sunlit side being visible from Earth
• Waxing means increasing
• Waning means decreasing
• Gibbous means that the amount of light we
see illuminating the Moon is between half lit
and fully lit